Financial wrangling over payouts from the September 11 World Trade Centre attack have been finalised, with insurers offering an additional $US2 billion ($2.44 billion) as the final step to rebuilding Ground Zero.

Seven insurance companies, including Allianz, Swiss Re and Zurich, have agreed to provide site developer Larry Silverstein with the extra payment, ending a five-year legal battle over the definition of a single terrorist attack.

Mr Silverstein had argued the attacks on the twin towers were separate events; some insurers defined them as a single incident, thereby limiting their exposure.

Courts ruled in October that payouts will vary depending on the wording of policies, with Mr Silverstein being awarded $US4.68 billion ($5.71 billion) of the $US7 billion ($8.54 billion) he sought.

Finalising the insurance payouts has delayed reconstruction at the site, of which the $US3 billion ($3.66 billion) Freedom Tower will form the centrepiece.